In order to improve the data retention characteristics for nonvolatile memory device, a high-quality inter-polysilicon polyoxide film is an indispensable dielectric material in terms of low leakage current and high electric breakdown field. So far polyoxide layers formed on polysilicon by high temperature oxidation with furnace or rapid thermal oxidation (RTO) still suffer defects of high leakage current and low electric breakdown field. The inventors of the present invention believe that these defects are brought about by non-uniform thickness of the polyoxide layer and the asperity at the polysilicon/polyoxide interface, which lead to the seriously enhanced localized electric field. The non-uniform thickness and asperity at the polysilicon/polyoxide interface problems are caused by different growing rates between the grain boundary and the grain bulk of the polysilicon layer in the high temperature oxidation process.
U.S. Pat. Nos. 5,616,233 and 5,736,454 disclose methods for forming a silicon dioxide layer on a single-crystal silicon substrate comprising conducting an electrolytic reaction at a room temperature such that a silicon dioxide layer is formed on a single-crystal silicon substrate acting as an anode. It was noted that the anodic oxide has the merit of less pinholes due to the self-readjustment nature of current conduction through the weak path in oxide during anodization. Therefore, it is believed that the anodic oxides should exhibit better oxide thickness uniformity and improved breakdown characteristics than the conventional thermal oxides as a gate oxide.